The present invention relates to cellular communication systems, more particularly to communication systems that employ Orthogonal Frequency Division Multiplexing (OFDM), and even more particularly to the reception of an OFDM signal in the presence of interference.
Cellular communication systems are well-known and are in wide-spread use around the world. FIG. 1 is a diagram illustrating a common feature found in most systems: a serving node 101 (depending on the system, it can be called a “base station”, a Node B, an evolved Node B (“eNodeB” or “eNB”)) serves user equipment (UE) 103 that is located within the serving node's geographical area of service, called a “cell” 105. For convenience, the term eNB will be used henceforth throughout this document, but any such references are not intended to limit the scope of the invention to only those particular systems that use this particular terminology. Thus, references to “eNB” are intended to also refer to “base stations”, “Node B's”, and “eNodeB's” and also to any equivalent node in a cellular communication system.
Communication is bidirectional between the eNB 101 and the UE 103. Communications from the eNB 101 to the UE 103 are referred to as taking place in a “downlink” direction, whereas communications from the UE 103 to the eNB 101 are referred to as taking place in an “uplink” direction.
To achieve bidirectional communications, a number of different strategies can be employed. For example, uplink and downlink transmissions can be scheduled to take place at different times in a so-called time division duplex (TDD) mode of operation so that they never occur at the same time. TDD operation permits, but does not require, the same frequencies to be used for both downlink and uplink transmissions.
An alternative strategy, called frequency division duplex (FDD) involves allocating different frequencies for downlink transmissions than are allocated for uplink transmissions. This permits, but does not require, downlink and uplink transmissions to occur simultaneously.
Because of the freedom afforded by FDD operation, many such systems schedule downlink and uplink data transmissions independently of one another, so that sometimes only downlink transmission is taking place, sometimes only uplink transmission is taking place, and sometimes both downlink and uplink transmission are taking place at the same time.
Aspects of a typical diversity receiver 200 for such a system (e.g., a Long Term Evolution—“LTE”—UE receiver) are depicted in FIG. 2. Although such a UE is said to use one transmitter and two receivers, Release 8 of the LTE standards permits a UE architecture to have only two physical antennas, and this will likely be the case for many practical embodiments. For two antenna architectures, the transmitter (TX) and one receiver (RX) branch share one of the same antenna 201 as shown in the figure. The receiver branch that shares the antenna with the transmitter branch includes a low noise amplifier (LNA) 203 for boosting the power of the received signal. The transmitter branch includes a power amplifier 205 for boosting the power of the signal to be transmitted. A so-called duplex filter 207 is then used to suppress leaking of the TX signal into the RX branch to a certain extent. (The illustrated amount of suppression is denoted atx-rx in FIG. 2, with the receiver part of the duplex filter 203 suppressing by an amount arx.) For example, the minimum requirements on the receiver's reference sensitivity (REFSENS) are defined in RAN4, see 3GPP TS 36.101, which basically verify the noise factor of the receiver.
In systems such as the LTE mobile communication system described above, there is a disturbance leakage between the uplink and downlink chains when the UE is scheduled to perform both uplink and downlink transmissions in the same subframe. This is due to second order inter-modulation products (IM2) in which unwanted TX signal leakage into the receive branch results in noise components around the demodulated DC sub-carrier. As shown in FIG. 3, the IM2 noise spectrum 301 is symmetric around DC and has a bandwidth that is approximately twice as large as the bandwidth of the uplink transmission. This leakage essentially increases the noise floor for the affected sub-carriers. This in turn can degrade the overall downlink demodulation performance as illustrated in FIG. 4, which shows the effect of degrading one third of the allocated sub-carriers of an exemplary system. The signal-to-noise ratio (SNR) of the sub-carriers experiencing the interference is varied between the curves.
In principle, the problem with IM2 products in the receiver can be solved by increasing the second-order intercept point (IP2) of the receiver (i.e., solved by imposing higher linearity requirements in the low noise amplifier and mixer components). However, achieving that linearity means increased cost and current consumption. Nonetheless, the sensitivity performance is an important measure of performance for a UE because it is related to coverage and is therefore an important aspect not only from a practical point of view, but also as a marketing tool. Hence linearity in the radio components is a trade-off between cost and performance.
There are also other IM products that can degrade the receiver performance close to the reference sensitivity level. For example, interference from third-order intermodulation (IM3) products originates when third order non-linearities cause the TX signals to mix with a strong blocking signal transmitted at a location in the radiofrequency spectrum that is substantially half the duplex distance between the transmitted and received signals.
In the above discussion, IM2 and IM3 are mentioned specifically. However, as is well known by those of ordinary skill in the art, higher order even and odd nonlinearities, respectively, will give rise to similar effects. Thus, for the sake of simplicity but without loss of generality, only IM2 and IM3 are mentioned here.
There is therefore a need for methods and apparatuses for improving receiver performance close to the reference sensitivity level when receiver impairments such as those discussed above are present.